Discoglossus Populations

Renet, Julien, Duguet, Rémi, Policain, Mathieu, Piquet, Alison, Fradet, Vincent, Priol, Pauline, Deso, Grégory, Grimal, François, Sotgiu, Giuseppe & Vences, Miguel, 2020, Alien populations of painted frogs, genus Discoglossus, on the southeastern coast of France: two examples of anthropogenic introduction, Amphibian & Reptile Conservation (e 266) 14 (3), pp. 189-199 : 192-195

Genetic Identity of the New Discoglossus Populations

The Maximum Likelihood tree reconstructed from the 258 bp cytochrome b segment retained for analysis ( Fig. 3 ) recovered phylogenetic relationships among Discoglossus species which were largely similar to those of more comprehensive, multi-gene studies ( Zangari et al. 2006; Pabijan et al. 2012; Biton et al. 2013; Dufresnes et al. 2020). However, as expected from such a short gene fragment, the relationships among most species were not reliably resolved. All species of Discoglossus were recovered as monophyletic groups, with bootstrap supports of 90–99%.

For the new continental French populations that are the focus of the present study, the tree is unambiguous in placing the samples from Marseille into the D. sardus clade, and the samples from Grimaud into the D. pictus clade. All nine D. sardus specimens sequenced from Marseille had identical sequences, and the same haplotype was also found in two localities in Corsica. In contrast, all sequenced specimens from the Hyères Archipelago and from Sardinia differed by at least three mutations, suggesting that the Marseille population most likely originated by the introduction of only a few individuals from Corsica. Of the two specimens from Grimaud, one had a haplotype identical to that of a specimen from Banyuls-sur-Mer, while the second one differed by a single mutation. This suggests a probable origin of this population by introduction from the invasive range of D. pictus in the southwestern French Mediterranean region.

The tree generated here also recapitulates the surprising finding of Zangari et al. (2006) regarding the presence of rather distinct mitochondrial haplotypes of D. pictus in Sicily. It also reveals that D. sardus from Corsica and Sardinia are not reciprocally monophyletic based on mitochondrial DNA. In this latter case, the one Corsican D. sardus (from Col d’Eustache) clustering among the Sardinian haplotypes was sequenced several months before the samples from Sardinia were processed, which excludes the possibility of an artifact due to a mislabelled sample or contamination.

Geographic Origin and Status of the New Discoglossus Populations

The results of this analysis shed a new light on the ranges of the two species of Discoglossus that are present in the south of France. In the population of D. sardus established in Marseille represents the second known mainland population, after the population of Monte Argentario peninsula (Tuscany, Italy). The genetic similarity of the Marseille samples with those from the two Corsican localities allow us to refute the hypothesis of an ancient relict population naturally occurring in Marseille. In such a case we would expect genetic relationships with the individuals from Port-Cros or the Levant islands (Hyères Archipelago), which are geographically much closer to Marseille than Corsica. On the contrary, an introduction from Corsica is consistent with the intensity of the maritime traffic between Corsica and Marseille, and the apparent lack of genetic diversity in the Marseille samples is also in agreement with an introduced origin. Even if the date of introduction of this species cannot be determined, its spread over an area of approximately 0.66 km 2 suggests that the arrival of D. sardus in Marseille is not very recent.

With a geographic extension of almost 210 km to the east (i.e., the distance between the easternmost population known to date and the recently discovered population at Grimaud), and its crossing of the Rhone River, the anthropic introduction of D. pictus is beyond doubt. Its arrival in Grimaud, along the La Garde river, could be linked with the trade activities of the many nurseries and garden stores (12 shops identified within 10 km of the sampling locality), which are known to be vectors of various species introductions worldwide (e.g., anurans, snails, plants; Christy et al. 2007; Bergey et al. 2014).

The two newly detected introductions of Discoglossus in continental France could have been accidental, or they could have been deliberate due to a variety of motivations, such as experimental studies on naturalization conducted in the past, or the liberation of captive animals. For instance, D. sardus tadpoles from Port-Cros Island were introduced into a tributary of la Mole river (Var) as an experiment in 1955, and this attempt at establishing a reproducing population is known to have succeeded at least until 1959 (Knoeppfler 1962).

The discoveries of these new populations testify once again that today the natural elements, such as rivers or oceans, do not represent absolute barriers for either native or allochthonous species. Invasion success generally depends more on the ability of a species to respond to natural selection than on broad physiological tolerance or plasticity ( Lee 2002). In the present case, considering the ranges of these two species and their reproductive status, it seems that they can be considered as successful colonizers. In fact, more comprehensive phylogeographic studies of D. pictus and D. sardus in the future should also examine the possibilities of D. sardus translocations among Corsica and Sardinia (given the clustering of the one Corsican haplotype among the Sardinian haplotypes; Fig. 3 ) and of D. pictus to or from Sicily (given the presence of highly distinct haplotypes on this island; Fig. 3 ).

Conservation Issues

Williamson (1996) considers that a biological invasion occurs when an organism takes root outside of its indigenous range. The IUCN Invasive Species Specialist Group proposes a more specific definition—that a biological invasion has occurred as soon as an introduced species is a factor of damage and affects the local biodiversity. In fact, it is important to distinguish between an allochthonous species introduced by humans, which is inoffensive in many cases, and an invasive species, which, by definition, is not only introduced outside of is indigenous range but also exerts a negative impact on biodiversity and more globally on the ecosystem ( Lambertini et al. 2011).

In the urban and sub-optimal ecological context of the city of Marseille, the population of D. sardus probably does not represent a threat to the ecosystem, which is a priori of ‘low ecological value.’ Furthermore, this population is already threatened by a large-scale urban development project. Although D. sardus is considered to be Least Concern in both the IUCN Red List and the National French Red List ( Andreone et al. 2009; UICN France et al. 2015), the global assessment has determined a decreasing population trend. This points to an important and challenging dilemma highlighted by Marchetti and Engstrom (2016): how to manage allochthonous, or even invasive species, that are threatened (or may become threatened in the future) in their native range? Several authors (e.g., Marris 2014; Heise 2018) have suggested pragmatic approaches when dealing with non-native species, especially in urban environments which indeed could become sanctuaries for many species (native or not) that are threatened in their original habitat. Especially with shifting ranges due to climatic change, the distinctions between native and non-native will become increasingly vague, and human-aided translocations of some threatened species are already being discussed ( Egan et al. 2018).

These elements lead us to consider the presence of these new Discoglossus populations as a high-priority conservation issue. We can also add that D. sardus is assessed as Threatened in the Var Department (cat. VU IUCN Redlist) [ Marchand et al. 2017], and as threatened with extinction at Port-Cros Island, Port-Cros National Park ( Duguet et al. 2019).

Concerning D. pictus , the question of its biological status requires more scrutiny because other authors have attributed an invasive nature with a high rate of dispersal to this species ( Montori et al. 2007). Its invasive capacity does not seem to be related to its adaptive advantages, but rather to the suitability of local abiotic conditions (Escoriza et al. 2014). The modeling of its potential habitat conducted by Escoriza et al. (2014) includes areas that are geographically near the locality of Grimaud, and incorporation of the new occurrences should allow an adjustment of the predictive models. Furthermore, the potential area of this species should be considered as wider than suggested by previous models. In any case, the expansion of D. pictus from a single location in Banyuls-sur-Mer, Eastern Pyrénées a century ago (see Wintrebert 1908) is not an artifact; i.e., it represents a natural range expansion ( Pujol-Buxó et al. 2019a) into a currently occupied area in France and Catalonia of more than 10,000 km 2 ( Montori et al. 2009). A negative impact of this species on co-occurring anurans (e.g., Pelodytes punctatus and Epidalea calamita ) has been suspected ( Escoriza and Boix 2012, 2014; Richter-Boix et al. 2013; San Sebastián et al. 2015). However, this possibility requires further study as some have hypothesized that temporal or evolutionary changes may have moderated the effects and disturbance of D. pictus on native species ( Pujol-Buxó et al. 2019b).

In any event, according to the actual current French regulations, all individuals of both species, as well as their “core” habitat, are strictly protected by a ministerial order (DEVN0766175A). Although a recent update of this order would specifically exclude D. pictus , we hope for the continued regulatory protection of D. pictus in French territory. Given the similarities in biotic features between the source and recipient communities ( Escoriza and Ruhí 2016), we suspect that Discoglossus species are probably not harmful to the local French anuran communities, and we therefore do not recommend the eradication of their non-native populations.

Lastly, to better manage this situation going forward, we recommend a monitoring program to: (1) characterize a predictable range expansion of these two painted frog species in adjacent localities; and (2) implement complementary studies in order to better assess the nature of the relationship between these introduced species and the native amphibian communities.

Acknowledgements.— Fieldwork and sampling were permitted by the French Government (permits n° 2017- 68/PJI and by prefect order of the Bouches-du-Rhône). We warmly thank the two reviewers, Daniel Escoriza and Sebastiano Salvidio, for providing useful comments. We also would like to thank Marin Marmier for his field assistance, and Giacomo Rosa for the revision of the English text.

Figures

Fig. 3. Maximum Likelihood tree of Discoglossus based on a 258 bp fragment of the mitochondrial cytochrome b gene. Numbers at nodes are bootstrap values (500 pseudoreplicates) in percent. After the locality, sample numbers are given, including GenBank accession numbers in parentheses for those sequences taken from GenBank. “Z” marks sequences from the work of Zangari et al. (2006). Samples from the two newly discovered introduced populations are highlighted in bold, red font.